It is known that many oxygen sensitive products, including food products such as meat and cheese, smoked and processed luncheon meats, as well as non-food products such as electronic components, pharmaceuticals, and medical products, deteriorate in the presence of oxygen. Both the color and the flavor of foods can be adversely affected. The oxidation of lipids within the food product can result in the development of rancidity. These products benefit from the use of oxygen scavengers in their packaging.
Some of these oxygen scavengers, typically unsaturated polymers with a transition metal catalyst, can be triggered or activated by actinic radiation. Such materials offer the advantage of an oxygen scavenger that does not prematurely scavenge oxygen until such time as the user decides to use the oxygen scavenger in a commercial packaging environment. The oxygen scavenger is thus “dormant” until it is passed through a triggering unit, typically a bank of UV lights through which an oxygen scavenger in the form of a film is passed to trigger the oxygen scavenging activity of the material. This is usually done just prior to a packaging step, in which a package having as a component the oxygen scavenger is made, with an oxygen sensitive product placed in the package prior to closure of the package to extend the shelf life of the oxygen sensitive product.
Unfortunately, triggering of the type of oxygen scavenger just described during high speed packaging applications (having a packaging line speed of greater than about 40 feet per minute) presents a challenge with conventional equipment and technology. Conventional triggering units, using current film formulations when triggered on Model 4104™ SIS units available through Cryovac, Inc., are typically limited in triggering speed to about 20 fpm or less. Conventional triggering equipment is already quite large, and to achieve higher speeds with this technology would require even larger units. The size and associated costs of purchasing or leasing such equipment, maintenance costs, and the requirement of space in the processing plant to accommodate such equipment, can be economically unattractive. Some processors or potential users of oxygen scavenging film do not have room for large equipment.
A class of oxygen scavengers which do not require actinic triggering, and thus do not require the associated triggering equipment, are iron based scavengers and some polymeric scavengers that are provided in active form. Such oxygen scavengers are active at the time of manufacture without triggering by actinic radiation. These also have several disadvantages, however. Some require the presence of moisture to initiate oxygen scavenging. This may not be technically attractive in packaging environments where it is otherwise undesirable or impractical to provide a moisture source to trigger the oxygen scavenger. Also, optics of the finished package can often be undesirably compromised by discoloration or pigmentation of the oxygen scavenger itself, either in its original state or after a period of oxygen scavenging activity. Processing of these oxygen scavengers in a uniformly dispersed way can also prove difficult in conventional extrusion operations. If such scavengers are not dispersed, as is the case with iron based sachets, scavenging activity may be too localized, and uniformity of scavenging may thus not be sufficient in the entire package environment to provide proper and adequate removal of oxygen from the head space of the package, and/or active barrier from subsequent ingress of oxygen from outside the package. Furthermore, oxygen scavenging sachets are unsuitable for vacuum packaging applications.
It is currently conventional practice to employ a non-migratory photoinitiator, such as those disclosed in U.S. Pat. No. 6,139,770, which is incorporated herein as if set forth in full. An example of a non-migratory photoinitiator is BBP3 (1,3,5-tris(4-benzoylphenyl)benzene).
It has now been found that certain blends of photoinitiators can be incorporated into one or more layers of an oxygen scavenging film, or into an oxygen scavenger composition or article, to improve the speed of triggering compared with that of conventional single photoinitiators. Certain blends of photoinitiators (PI) have been found to increase the speed at which oxygen scavenging film can be triggered by as much as two times. The selection of suitable blends depends in part upon the spectrum of the UV lamp used to trigger the film. Photoinitiators can be selected that absorb longer wavelength UV and/or visible light. Such photoinitiators allow for a greater portion of the emission from a polychromatic source to be utilized. Furthermore, triggering of an oxygen scavenger can be accomplished through materials that may be partially or completely UV opaque by utilizing the visible spectrum.
It has thus been found that the addition of certain photoinitiators to a conventional formulation with BBP3 can result in film that can be triggered at higher speed. With current germicidal lamps (254 nanometer principal emission wavelength), benefit was seen with the addition of isopropylthioxanthone (ITX) to BBP3, with triggering speeds increased from 20 fpm to 40 fpm. Commercial grades of ITX are known to be a mixture of isomers, principally 2 and 4 substituted. Other thioxanthone derivatives such as 2,4-diethylthioxanthone, 2-chlorothioxanthone, and 1-chloro-4-propoxythioxanthone can also show this enhancement. The addition of 4,4′-benzoylmethyl diphenyl sulfide (BMS) photoinitiator showed some benefit with all lamp types. Other substituted diphenyl sulfides can also show this benefit. When a polychromatic UV source was used such as a medium pressure mercury arc lamp or a xenon flash lamp, benefit was seen with the addition of IRGACURE™ 819 (bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide). Other acylphosphine oxide type photoinitiators such as 2,4,6-trimethylbenzoyidiphenylphosphine oxide, ethyl-2,4,6-trimethylbenzoylphenyl phosphinate, and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide can show a similar effect. Beneficial levels of PI to add to the conventional formulation were found to be in the range of from 500 to 1000 ppm.
Higher triggering speeds can be useful for vertical and horizontal pouch machines, which typically operate at speeds of 40 to 80 fpm.
The present invention also allows the triggering of film formulations at e.g. about half the typical dose of irradiation. A combination of faster triggering and lower dose can also be achieved in some cases.
These improvements are with respect to the present use of BBP3 in conventional oxygen scavenging film formulations, and using currently commercial triggering equipment. However, improvements can also be realized using alternative triggering sources as well.